The present invention is generally directed to processes for obtaining ink donor films; and more specifically the present invention is directed to an emulsion coating method for generating thermal transfer ink donor sheets. Therefore, in one embodiment of the present invention there is provided a process for thermal ink donor films by causing the grinding of a formed emulsion containing the appropriate components, and thereafter heating to remove the solvents present enabling the effective dispersion of pigment particles in a polymer or wax composition. The sheets obtained in accordance with the process of the present invention are useful in known thermal transfer printing methods, such as those described in Thermal Transfer Printing: Technology, Products, Prospects, Published by Datek Information Services, P.O. Box 68, Newtonville, Mass.; or in the IBM Journal of Research & Development, Vol. 29, No. 5, 1985, the disclosures of which are totally incorporated herein by reference.
Thermal printing is a nonimpact process that permits the formation of images of excellent resolution; and further these processes are simple in design, offer low undesirable noise levels, and are very reliable over extended usages. Two classifications of the aforementioned thermal printing processes are direct thermal printing, and thermal transfer printing. In the direct method there are selected special papers coated with heat sensitive dyes, while in the transfer method an intermediate sheet is initially coated with a pigmented layer from which certain areas thereof are transferred to a receiving substrate to provide a final printed image. Direct thermal printing is not preferred in that, for example, the prints resulting are subject to fading, and thus have poor archival characteristics. Also, it is known that the direct thermal transfer papers are of an aesthetically objectionable appearance. In contrast, with thermal transfer printing methods there are used uncoated plain papers enabling prints with acceptable appearance, and excellent archival properties. Another advantage of the thermal transfer printing method resides in its applicability to color printing wherein there can be selected multicolored ink donor films.
In the aforementioned thermal transfer printing processes there is selected a thermal print head, an ink donor film comprised of thin paper, or a plastic film coated with a solid heat fusable ink, and a plain paper receiver sheet. To obtain final prints there is placed in contact with the receiver sheet the ink side of the donor film, followed by the application of heat originating from the print head to the film. Heat conducted through the donor film increases the temperature of the ink to above melting permitting the ink to wet the receiver sheet, and finally resolidify thereon. Since the receiver sheet is wet by the molten ink, and is rougher in texture than the smooth donor film base, the resolidified ink preferentially adheres to the receiver. Subsequent to separation of the receiver sheet from the ink donor film, there results transferred ink on the imaged areas of the receiver. Alternatively, as described in the aforementioned IBM Journal disclosure, the heating may be generated by passage of an electric current through a resistive layer which is an integral part of the film.
More specifically, the ink donor films selected in the prior art are generally comprised of a thin base film, such as glassine, condenser paper, or polyester substances with a coating thereover of pigmented wax or polymer. The supporting substrate, or smooth base film is from about 5 to about 20 microns in thickness, and is of sufficient strength to permit use thereof without tearing. Since the primary function of the substrate is to transport heat from the printhead to the ink layer, its properties should be designed to the extent that it has high intrinsic thermal conductivity. Further, the sheet selected should be thin, that is for example about 20 microns or less, and smooth to allow for the more effective transfer of heat. Additionally, the substrate sheet is formulated in a manner to withstand the high printhead temperatures, about 300 degrees Centigrade, for several milliseconds without melting or charring.
Donor ink compositions presently utilized are comprised of pigmented waxes or low molecular weight polymers, which are generally blended to permit the formation of a film. Blending is affected to generate a reasonably hard film with a low melt viscosity when subjected to heat of a temperature of from 60 to 80 degrees Centigrade originating from the printhead. Thus, normally the film is formulated from the blending of hard paraffin waxes with softer ester waxes. Also, the wax with pigment therein is coated on the substrate by, for example, gravure or roll coating methods.
Other important characteristics associated with ink donor films, which characteristics are achievable with the process of the present invention, include substantial adherence of the final ink film to the donor film base. Moreover, the ink donor film should be of sufficient resiliency to enable reasonable deformation without flaking; and additionally, possess a temperature viscosity profile permitting a relatively sharp decrease in the viscosity of the ink donor film at its melting point. Additionally, the coating selected for the aforementioned ink donor films should be substantially abrasion resistant to prevent transfer of the coating to the contact receiving sheet. Furthermore, there is desired ink donor films that permit smear resistant images subsequent to transfer to other substrates. Also of importance is that the ink layer selected should have a low degree of surface tackiness to prevent adherence to itself at normal operating temperatures when subjected to light rolling, an object achievable with the invention of the present application.
Many of the aforementioned desirable characteristics require the use of a variety of additives to the ink layers including, for example, oils and plasticizers. These additives, which are selected to modify the mechanical properties of the film, for example, include specifically high molecular weight polymers such as cellulosic derivatives for controlling melt viscosity and tensile strength, and commercially available ethylene-vinyl acetate copolymers which are used to modify adhesion properties and surface tackiness. Further, pigment dispersants known in the art can be incorporated into the ink donor films to increase the optical density thereof.
Presently, most ink donor films are prepared by melt blending pigments, and optional additives dispersed in a wax, followed by the hot melt coating of the resulting ink on a suitable substrate, such as Mylar or condenser paper. Solution coating is not feasible for obtaining the donor sheets in view of the solubility characteristics of the ink components, that is for example the waxes are insoluble in common commercially available solvents at normal temperatures. Further, while hot melt blending and coating processes are satisfactory for generating ink donor sheets of reasonably uniform properties, minimum dispersion of the pigments occur because of the poor wetting characteristics of the wax. Also, the high solids content, essentially 100 percent, of the melt creates substantial difficulties in controlling the coating parameters, such as viscosity and leveling. The use of the commercially available additives indicated hereinbefore to control properties such as pigment dispersion, surface tension, tensile strength, melt viscosity, leveling, and smear and abrasion resistance is restricted to materials that are soluble or readily dispersible in the wax melt, and which do not otherwise adversely effect the hot melt coating properties.
Thermally activated inks and transfer ribbons are illustrated in U.S. Pat. No. 4,503,095, the disclosure of which is totally incorporated herein by reference. Specifically, there is described in this patent, a thermally activated medium which can be used for color printing, which contains a multiplicity of inks such as cyan, magenta and yellow ink compositions applied side by side on a substrate. The ink selected comprises a wax blend, softening agent, pigment, extender pigment and heat-conductive powder which is applied to the substrate as a hot melt. Other prior art includes U.S. Pat. No. 3,970,002 directed to inks comprising dye-wax-oil compositions which are also coated by hot-melt methods. Further, U.S. Pat. No. 4,308,318 discloses the preparation of thermally activated transfer ribbons for nonimpact printers in which the thermal ink layer is prepared by organic solvent-based coating and comprises a polyamide, and pigment and dye dispersed in propyl alcohol. In addition, there is disclosed in U.S. Pat. No. 4,251,276 similar formulations using a mixed solvent system as the coating vehicle. Moreover, in U.S. Pat. No. 3,336,150 there is disclosed an impact copying sheet and wherein, for example, there is described a process for suspending or dispersing colorants in a liquid vehicle, dissolving a carrier resin or wax in a solvent, and thereafter combining the aforementioned materials by ball milling, followed by solvent evaporation. However, there is no disclosure in the prior art relating to the preparation of ink donor films by dissolving the carrier wax or polymer in a liquid vehicle, dispersing the colorant and optional additives in a different liquid, and combining the two immiscible materials by ball milling, followed by coating onto a substrate as achieved with the process of the present invention. Additionally, references of background interest include U.S. Pat. No. 2,499,004; 3,957,495; and 4,407,886; European Patent Applications 63,000 and 82,270; Japanese Patent Publication 33174 (284); and IBM Disclosure, Volume 27, Number 3, pages 1806 to 1807.
Although the above hot melt blending and homogeneous solution processes for generating donor film compositions are suitable for their intended purposes, there continues to be a need for new processes. Additionally, there continues to be a need for formulation methods that will enable ink donor films with uniform characteristics, and wherein the pigments are more completely dispersed in the waxes or polymers selected. Further, there is a need for donor film processes wherein melting the wax component to permit coating is eliminated. Also, there is a need for ink donor film formulation methods that will provide ink composition coatings of a substantially uniform thickness thereby enabling the generation of uniform color prints. Moreover, there is a need for processes for donor films that are relatively simple, and wherein the solvents used are recoverable, and can be recycled for repeated usage. In addition, with the process of the present invention dispersing agents can be avoided; and also the use of a polar dispersing media permits the incorporation of optional wax incompatible additives, such as alcohol soluble dyes, and surface active or viscosity controlling polymers, including cellulosic derivatives. Additionally, with the process of the present invention, the use of other known additives for the purpose of controlling properties such as surface tackiness, tensile strength, adhesion characteristics, and abrasion resistance is simplified compared to prior art processes. Furthermore, the emulsion dispersions formulated in accordance with the process of the present invention are somewhat thixotropic allowing stability during storage. Moreover, the process of the present invention permits the coating of the ink composition to be accomplished at room temperature.